Cryogenic air separation has been the major commercial source of air components for over half a century, and those components have become increasingly important industrial products. For example, steelmaking today is dependent upon pipe-line quantities of oxygen.
Conventionally, cryogenic air separation is carried out in a system of stationary distillation columns which may be a hundred or more feet tall and ten or more feet in diameter. Within the column, liquid flows downwardly be gravity into and out of a series of plate-like structures holding shallow pools of liquid. The bottoms of the plates or trays are perforated so that vapor under sufficient pressure flows upwardly into and through each liquid pool to the next higher pool. The vapor pressure is also sufficient to prevent the liquid pools from "weeping" through the tray perforations. The liquid must go downward by gravity from one pool to the next via drains called downcomers. The result is a stage by stage counter-current contact between the liquid as it moves down the column and the vapor as it moves upwardly. Alternatively, instead of trays, the distillation columns are filled with a mass of material referred to as packing, which can be, for example, glass beads, shaped metal pieces, wire mesh or a honey comb-like structure made of sheet metal. Liquid flows down the labyrinth of passages in the packing and makes continuous counter-current contact with the vapor which rises because a somewhat lower pressure is maintained at the top of the column than at the bottom. With packing, the vapor is not "forced" through successive pools of liquid but passes over the surface of the liquid which is in the form of a downwardly flowing film or streamlet. Consequently, less difference in pressure is required between the bottom and the top of a column in operating a packed column than a trayed column. This can be translated into lower energy costs for a packed column.
In a stationary cryogenic air separation system, the separation process or rectification is basically the same whether the columns are trayed or packed. How the separation takes place is illustrated by separation of nitrogen from oxygen in the largest column of a typical system, called the low pressure column. Liquid nitrogen boils at lower temperature (-383.degree. F.) than liquid oxygen (-360.degree. F.), and therefore liquid nitrogen is said to be more volatile (capable of being changed from a liquid to a gas) than liquid oxygen. The boiling points given for both liquid nitrogen and oxygen are at one atmosphere of pressure which is normal atmospheric pressure at sea level. In a typical low pressure column, the downwardly flowing liquid begins at the top as nitrogen. The rising vapor at the bottom is oxygen from a boiling pool. The oxygen vapor, as it rises, picks up the more volatile nitrogen from the descending liquid and becomes increasingly richer in nitrogen. At the top of column, the vapor becomes nearly 100% nitrogen, one product of the distillation, and it is in equilibrium with the liquid nitrogen which in turn is "stripped" of almost all of its nitrogen as it descends and becomes nearly 100% oxygen, another product of the distillation, at the bottom of the column. Air is fed to the middle of the column.
It is an object of this invention to carry out mass and/or heat transfer between two fluids of different densities, including cryogenic air separation, in a rotating apparatus using an ordered or structured packing mass wherein one fluid is expelled from the center of the mass uniformly through the packing by centrifugal force resulting from the rotation, and the other fluid, less dense than the first fluid, moves from outside of the mass through the packing toward the center.
It is another object of the invention to greatly reduce the size of a cryogenic air separation system from that required when conventional stationary distillation columns are utilized.